This invention relates generally to flexible printed circuits and, more particularly to flexible printed circuits with raised contacts.
It is well known in the art to use raised contact elements (sometimes referred to as "bumps") to make contact for electrical interconnection. Such bump contacts are frequently used on interconnect devices or circuitry that is used to connect a circuit to an electronic component (for example an integrated circuit or IC) or to another circuit. By way of example, it is known to use bump contacts to connect one circuit component to another, often by the use of a pressure connection system such as shown, e.g., in U.S. Pat. Nos. 4,468,074, 4,610,495, and 4,768,971 (which are assigned to the assignee hereof and fully incorporated herein by reference). For example, it is known to use contact bumps on flexible circuitry to connect two circuit boards via a pressure connection. In such a pressure connection system, electrical connection between the raised contact patterns of the circuit and the contact surfaces an IC is generally made by pressing the contact surfaces of the IC against the contact pads of the circuit. Reliability of this electrical interconnection is increased by use of the raised contacts. Raised contacts of the type described herein have been manufactured by either electroplating metal onto the pad surface or by deforming the substrate below the pad surface with a male/female tool. An example of an electroplating scheme for the formation of co-planar bumps is disclosed in U.S. patent application Ser. No. 601,904 filed Oct. 23, 1990 (now U.S. Pat. No. 5,072,520). When a male/female tool is used, gaps are often formed in the substrate which are then filled with an epoxy to prevent the raised contact pads from collapsing. In either case, control of the coplanarity of the raised feature is very important. The coplanarity of the raised feature must be maintained while under contact pressure. The raised pad surface is often deformed when the pressure connection system is applied. When the raised pad surface is deformed, registration between the pads and the contact surface of the IC or other device is degraded. Thus, a need exists for a flexible circuit which has raised contact pad features that will maintain registration between the contact pads on the circuit and the contact surfaces of the IC or other device when the pressure connection system is applied.
This uniformity of contact height is especially important when the density of bump contact increases and the bumps come closer together. In such cases the flexibility of the interconnect circuit material may not provide the needed compensation to overcome differences in bump height, and malfunctions will occur because of failure to make contact where required.
When bump contacts are used in a high density contact applications, such as multichip module (MCM) to circuit board connections (e.g., pitch (center to center of adjacent contact sites) under 50 mils), it is extremely important that coplanarity of the bump contact surfaces be maintained. That is, the height of the bumps must be carefully controlled so that the top or contact surfaces of all of the bumps will be in the same plane in order to make contact with the contact points on the MCM or other device to be contacted. If the bump height differs from bump to bump, contact may not be established at all contact sites. For example, if a shorter bump is located between two higher bumps, the shorter bump may not make contact with its intended contact site on the MCM or other device, thereby resulting in a malfunction.